Although the viral genome is often quite small, it encodes a broad series of proteins. The virus takes advantage of the host-RNAprocessing machinery to provide the alternative splicing capability necessary for the expression of this proteomic diversity. Serinearginine-rich (SR) proteins and the kinases that activate them are central to this alternative splicing machinery. In studies reported here, we use the HIV genome as a model. We show that HIV expression decreases overall SR protein͞activity. However, we also show that HIV expression is significantly increased (20-fold) when one of the SR proteins, SRp75 is phosphorylated by SR protein kinase (SRPK)2. Thus, inhibitors of SRPK2 and perhaps of functionally related kinases, such as SRPK1, could be useful antiviral agents. Here, we develop this hypothesis and show that HIV expression down-regulates SR proteins in Flp-In293 cells, resulting in only low-level HIV expression in these cells. However, increasing SRPK2 function up-regulates HIV expression. In addition, we introduce SR protein phosphorylation inhibitor 340 (SRPIN340), which preferentially inhibits SRPK1 and SRPK2 and down-regulates SRp75. Although an isonicotinamide compound, SPRIN340 (or its derivatives) remain to be optimized for better specificity and lower cytotoxicity, we show here that SRPIN340 suppresses propagation of Sindbis virus in plaque assay and variably suppresses HIV production. Thus, we show that SRPK, a well known kinase in the cellular RNAprocessing machinery, is used by at least some viruses for propagation and hence suggest that SRPIN340 or its derivatives may be useful for curbing viral diseases.HIV ͉ kinase inhibitor ͉ SR protein phosphorylation inhibitor 340 H IV-1 precursor RNA transcribed from proviral DNA integrated in the host cell genome contains all of the transcribed viral reading frames (1). Alternative splicing is essential for producing mRNAs encoding various viral proteins from the limited size of a single precursor mRNA (2). In the early phase of HIV expression, eight splice acceptor sites compete for the splicing machinery to produce the vif, vpu, vpr, nef, env, tat, and rev mRNAs (3). In the late phase of the virus life cycle, singly spliced longer RNA is translated to a polyprotein and then cleaved by HIV protease to generate gag and pol proteins. Several reports show that regulation of the complex splicing pattern can dramatically affect HIV-1 infectivity and pathogenesis (4-6). However, little is known about the molecular mechanism that links this alternative splicing regulation and the dynamics of virus propagation.Alternative splicing depends on the alternative utilization of four 5Ј splice sites and eight 3Ј splice sites (3). The combination of these splice sites are regulated by cis-regulatory elements, which bind cellular heterogeneous nucleoproteins (hnRNPs) of the A, B, and H groups and serine-arginine-rich (SR) proteins (7). SR proteins are highly conserved in eukaryotes and are characterized by having one or two RNA-recognition motifs at the amino termi...

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